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The Genome Evolution of Borrelia Species Spirochetes
Borrelia Background Borellia is a spiral (spirochete) genus of bacteria. They are considered to be a ‘double membrane’ bacteria, and neither gram-positive nor gram-negative. The Borellia species cause borreliosis, a zoonotic disease characterized by a fever, headache, and rash (1). The bacterium is transmitted primarily through bites, the Ixodes ''tick being the major host vector. The bacterium is taken up into a tick from the blood of a previously infected host, and then lives in the tick until it is transmitted into the blood of its next host. Certain ''Borrelia species can only infect certain wildlife populations; the bacterium may thrive in mice, squirrels, dogs, and birds, but cannot survive in others, such as deer (2). Up until the 1990s, it was through that only one bacterium caused borreliosis; it was then discovered that the Borrelia species was a complex of multiple bacterial species. To date, 36 individual species have been discovered or proposed; 7 species are present in North America. The specificity of pathogenicity in Borrelia species is determined by several factors: the host specificity, the geographic location, and the ability to cause disease in humans (1; 2). Introduction While arguably the most well known Borrelia name is Borrelia burgdorferi, many other Borrelia ''species exist. As of 2014, 20+ complete and draft ''Borrelia species' genomes have been published. While 36 individual species exist, not all Borrelia species cause Lyme borreliosis. While nearly 18 species are believed to cause Lyme disease, several others are considered to be the causative agent of relapsing fever disease, a more rare disorder than Lyme (2; 3). So what has caused such pathogenic diversity to develop among Borrelia species? On the large-scale level, researchers believe the answer is related to host specificity and vectors. Because some vectors are not ‘compatible’ with certain Borrelia ''species, this affects the global distribution of the bacterium; in addition, the migratory movements of certain hosts may determine how ''Borrelia ''is distributed globally. However, on the genetic level, researchers believe the answer is related to the complexity of the ''Borrelia genome. Because of ''Borrelia’s ''unique, repetitive linear chromosome, it is highly susceptible to mutations. According to researchers, horizontal gene transfer is extremely rare in the context of the ''Borrelia genome; genetic changes most often occur because of random deletions, duplications, recombination, and natural selection (2; 3). Results * The flaB ''gene (a flagellin gene) was found to be expressed in all relapsing fever ''Borrelia samples, but not expressed in any Lyme samples. * The ospA gene (an outer surface protein) was found to be highly variable in B. garinii strains, yet highly homogenous among other species. * The ospC gene (an outer surface protein on a circular plasmid) was found to be highly variable among all Borrelia species; it is believed to be associated with host-to-species specificity as well as tick-to-host transmission specificity. * A specific 16S-23S rRNA intergenic spacer was discovered to encode for loci only found in the North American B. burgforferi genome. * A specific 23S-5S rRNA intergenic spacer was discovered to encode for loci unique to Lyme borreliosis-causing Borrelia species. * The ospB gene (an outer surface protein) was discovered to be most highly expressed during mammalian host infections; this gene was found to be obsolete from B. garinii, a bird-adapted Borrelia species. * The PF54 gene (a surface lipoproteins gene) was found to be highly variable among all Borrelia ''species, adapted to evade the immune response generated by the infected host. Borrelia in North America In the context of North America, the most commonly encountered ''Borrelia species is Borrelia burgdorferi; more than 80% of B. burgforferi ''cases occur in the Northeast region of the US. While an expansive diversity of ''Borrelia species exist in the Southern and Western areas, a very low number of borreliosis cases are documented each year (2). Researchers have hypothesized that bird-vectors have spread certain Borrelia species from North America to both Europe and Canada, accounting for similarities between unique bacterial populations. However, many of the genes expressed in North American B. ''burgforferi ''strains are not expressed at all in European ''B. burgforfer''i strains; this can be attributed to local environmental pressures put on individual strains (2). The genomic evolution of North America ''Borrelia species has been fueled by differing long-term and short-term conditions, such as glacial-interglacial cycles, European settlers, and growing deer populations (2). Major Conclusions * The genes encoding major surface exposed antigens were found to be highly variable among nearly all Borrelia ''species. * The ''Borrelia ''genome has evolved primarily because of host migration and environmental influences. * The evolution of certain genes is determined on a case-by-case basis and does not necessarily reflect evolution of the entire ''Borrelia genome itself. References 1) Borrelia. Wikipedia. http://en.wikipedia.org/wiki/Borrelia. Updated July 27, 2014. Accessed November 17, 2014. 2) Margos G, Vollmer SA, Fish D, et. al. Population genetics, taxonomy, phylogeny, and evolution of Borrelia burgdorferi sensu lato. Infect Genet Evol. October 2011; 11(7): 1545-1563. doi: 10.1016/j.meegid.2011.07.022 3) Qiu W, Martin CL, et. al. Evolutionary genomics of Borrelia burgdorferi sensu lato; Findings, hypotheses, and the rise of hybrids Infect Genet Evol. October 2014; 27: 576-593. doi: 10.1016/j.meegid.2014.03.025